Neil: The intention is that only methods can be abstract.  I don't think we should attempt to only check the __isabstractmethod__ attribute for objects that we know are methods; that would be an expensive check to make (you'd have to call __get__ if it exists) and since this is a __magic__ attribute, you void your warrantee if you mess with it. :-)

In this version (v3), I've fixed the C nits and done the refactoring you suggested, and also added an optimization: check_abstracts() returns immediately if the base class doesn't have the ABSTRACT flag set.
File Added: abstract.diff